Device for measuring a load at the end of a rope wrapped over a rod

ABSTRACT

A machine for raising and lowering a load, the machine including a platform, a boom attached at one end to the platform, a rod mounted transverse to the longitudinal axis of the boom on the other end of a boom, the rod having a central bore there through, with at least two strain gauges mounted therein and angled relative to each other, a hoist rope extending over the rod and having an end adapted to be attached to the load, and a mechanism connected to the at least two strain gauges for determining the angle of departure of the hoist rope from the rod and the amount of pull of the load on the hoist rope.

BACKGROUND OF THE INVENTION

This invention is directed to devices for measuring the weight of a loadat the end of a rope wrapped over a rod.

It is known to measure the weight of a load at the end of a rope wrappedor trained over a rod by using four strain gauges configured as a bridgecircuit glued inside a central bore 3 in a rod 4. This is an optimumconfiguration to measure a force applied in a vertical direction only.When the force to be measured has a constant angle relative to thestrain gauge, one set of gauges has been used.

The calibration procedure used in this instance is as follows.

1. Place the instrumented rod in a calibration fixture that applies aload in the vertical direction.

2. Apply an increasing load in a series of load steps. The maximum loadis the expected maximum load the sensor will see. Measure the straingauge bridge output at each of these load steps.

3. Repeat step 2 for decreasing load steps to zero load.

4. A curve is then fitted to this data, for example, a simple linearcurve y=mx+b. Then for any given x, the output y can be calculated. Ifthe slope m has units of pounds/mV and assuming b=0 lbs., then we candirectly relate the output of the bridge circuit to the applied load P.A linear fit is known to give an overall accuracy of 1% full scale.

It is also known to use two strain gauge bridge circuits disposed tointersect each other at right angles to measure tensile or compressivestrains in biaxial directions. This is known as a bi-axial load-sensingelement.

SUMMARY OF THE INVENTION

It has been difficult to accurately calibrate strain gauges for aresultant force P with a non-stationary point of application. If theangle of this force changes with a change in rope wrap angle, i.e., ifthe rope pull can be from different directions, two sets of gaugesoriented perpendicular to each other are needed.

The invention eliminates these inaccuracies. This invention provides amachine for raising and lowering a load, the machine including aplatform, a boom attached at one end to the platform, and a rod mountedtransverse to the longitudinal axis of the boom on the other end of aboom. The rod has a central bore there through, with at least two straingauges mounted therein and angled relative to each other, a hoist ropeextending over the rod and having an end adapted to be attached to theload, and a mechanism connected to the at least two strain gauges fordetermining the angle of departure of the hoist rope from the rod andthe amount of pull of the load on the hoist rope.

The object of this invention is to provide a relatively accurate devicefor measuring the weight of a load at the end of a rope wrapped over arod, where the direction of rope pull caused by the load relative to therod is variable.

It is still another advantage of this invention to provide a measuringsystem of the foregoing type which can determine such calculations in adynamic state and with an accuracy of within + or −0.1%.

It is another object of this invention to provide an improved system formeasuring the load weight of large mining and lifting machinery, whichsystem eliminates the need to try to compute dynamic force effects onthe machine in order to get an accurate weight measurement.

It is yet another advantage of the present invention to provide ameasuring system of the foregoing type which is adaptable for use with awide variety of mining and lifting machinery.

It is yet another advantage of the present invention to provide a devicethat can automatically calibrate equipment, so the device accuratelydetermines the location of a load carried by a mining and liftingmachinery.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a partial broken away side view of a pin with a rifle borewith strain gauges mounted in the bore.

FIG. 2 is a plan view of a surface mining shovel that employs themechanism of the present invention.

FIGS. 3A, 3B and 3C are a perspective view of the sheave rod of thisinvention, with a cross section showing the placement of the two sets ofstrain gauges and the resulting strain gauge bridge and the mechanism ofthis invention.

FIG. 4 is a cross section of the sheave pin with arrows showing how todetermine the pull of a rope given the strain gauge outputs.

FIG. 5 is a cross section of the sheave pin and sheave with arrowsshowing how to determine the tension and angle of the rope wrapped overthe sheave on the sheave pin.

FIG. 6 is a graph of the sheave pin calibration data.

FIG. 7 is a graph of the calibration data showing how the data is notlinear.

Before one embodiment of the invention is explained in detail, it is tobe understood that the invention is not limited in its application tothe details of the construction and the arrangements of components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Also, it is to be understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. Use of “including”and “comprising” and variations thereof as used herein is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items. Use of “consisting of” and variations thereof as usedherein is meant to encompass only the items listed thereafter andequivalents thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described below and as illustrated in the drawings, the invention isa machine for raising and lowering a load, such as a hoisting machine 8including a device 22 supported from a structure. More particularly, themachine can be a piece of large mining machine or an industrial crane,or any device for lifting a load. In this detailed description, a powershovel is described, and the device 22 for holding the material to belifted is a dipper.

The power shovel 8 of FIG. 2 is of a well-known construction commonlyreferred to as an electric rope shovel. The shovel 8 comprises a mobilebase 10 supported on drive tracks 11, and having supported thereonthrough a turntable 12, a platform in the form of a machinery deck 13.The turntable 12 permits full 360 degrees of rotation of the machinerydeck relative to the base. A boom 15 is pivotally connected at a lowerend 16 to the machinery deck 13. The boom 15 is held in a upwardly andoutwardly extending relation to the deck by a brace in the form oftension cables 18 which are anchored to a back stay 19 of a staystructure 20 rigidly mounted on the machinery deck 13. A sheave 17 isrotatably mounted on the upper end of the boom 15.

The dipper 22 is suspended from the boom 15 by a hoist rope 23 wrappedover the sheave 17 and attached to the dipper 22 at a bail pin 30. Themachine structure is movable to locate the dipper 22 in respectiveloaded and unloading positions. And the hoist rope is anchored to awinch drum 24 mounted on the machinery deck 13. As the winch drumrotates, the hoist rope 23 is either paid out or pulled in, lowering orraising the dipper 22. The dipper has an arm (also known as a handle) 25rigidly attached thereto, with the dipper arm 25 slidably supported in asaddle block 26, which is pivotally mounted on the boom 15 at 27. Thedipper arm has a rack tooth formation thereon (not shown) which engagesa drive pinion (not shown) mounted in the saddle block 26. The drivepinion is driven by an electric motor and transmission unit 28 to effectextension or retraction of the dipper arm 25 relative to the saddleblock 26.

A source of electrical power (not shown) is mounted on the machinerydeck 13 to provide power to one or more hoist electric motors (notshown) that drives the winch drum 24, a crowd electric motor (not shown)that drives the saddle block transmission unit 28, and a swing electricmotor (not shown) that turns the machinery deck turntable 12. Theabove-described basic construction of the shovel loader is widely knownand used and further details of the construction are not provided, asthey are well known in the art.

Each of the crowd, hoist, and swing motors is driven by its own motorcontroller (not shown) which responds to operator commands to generatethe required voltages and currents in well known fashion.

More particularly, as shown in FIG. 1, the sheave 17 is mounted on asteel sheave rod 38. More particularly (not shown), there are two hoistropes and two spaced apart sheaves mounted on the sheave rod 38. Asshown in FIG. 1, each of the spaced apart sheaves 17 is positionedinside of grooves 45 in the sheave rod 38, with a portion of each sheaveoverlapping each groove. The pull of the hoist rope 23 is generallyequally divided between the sheaves, so, i.e., if there are two sheavesone half of the rope pull is experienced by each sheave. In otherembodiments (not shown), only a single hoist rope and sheave can beused. Hereinafter, any reference to a hoist rope or a sheave is alsointended to include 2 or more hoist ropes and sheaves.

As shown in FIGS. 1 and 3, the sheave rod 38 has a central rifle drilledbore 39, and two sets of two strain gauges 40 each, as shown in FIG. 3,are glued inside the bore 39. Each set of strain gauges is mountedadjacent one of the sheaves, and is responsive to the rope pull ortension seen by the sheaves, which in turn deflect the sheave rod. Theoutput of the two sets of strain gauges is combined through a bridgecircuit 43 to get the output used for weight measurement. An example ofthe bridge circuit for measuring forces in an X direction is shown inFIG. 3. One half of the bridge circuit is located under each sheave.

In a similar fashion, a second bridge circuit (see FIG. 4) for measuringforces in the Y direction is located in the same manner inside the bore39, only offset 90 degrees from the strain gauges of the first bridgecircuit. The output from these bridge circuits of the instrumentedsheave rod is used to determine the total applied force, as explainedbelow.

As shown in FIG. 4, the cross section of the bi-axial sheave pin 38illustrates the gauge alignment. The gauges marked Y sense loadcomponent Py, while the gauges marked X sense load in the Px direction.Given the voltage outputs form the two bridges as Ex and Ey andconstants Kx and Ky derived from calibration data, as hereinafterexplained the resultant force P is determined by Px=Kx*Ex and Py=Ky*Eyand the resultant Force P equals the square root of the same of thesquares of Px and Py. As shown in FIG. 5, the line tension T and wrapangle Θ can be determined by T equaling the sum of Px² and Py² dividedby 2Px, and Θ equal to 2 cos⁻¹ P divided by 2T.

The power shovel 8 also includes means in the form of a strain gaugereading mechanism 50 connected to the two sets of strain gauges 40 fordetermining the angle of departure of the hoist rope 23 from the sheaverod 38 and the amount of pull of the load on the hoist rope 23, themechanism 50 being, as shown schematically in FIG. 3, in the form of acentral processing unit 54 including software 56 and memory 58, thememory including strain gauge calibration information. The mechanism 50uses a method of determining the tension or force and angle of the hoistrope 23 wrapped over the sheave rod 38. The method comprises thefollowing steps. First, of creating a calibration table of a pluralityof outputs from each of the at least two strain gauge bridge sets, givenvarious rope tensions and rope angles. Then second, when encountering anunknown rope tension and rope angle, taking the output from one straingauge set and creating a set of data point angle and tension pairs fromthe plurality of calibration outputs that correspond to the one outputfrom the one strain gauge set, and taking the output from the otherstrain gauge set and creating a set of data point angle and tensionpairs from the plurality of calibration outputs that correspond to theone output from the other strain gauge set. Lastly, then determining,based on where the two sets of data points intersect, the rope tensionand rope angle.

For a given output from the strain gauges, an amount of pull anddirection of pull can be determined to within a+ or − accuracy of 0.1%,as further described below.

In other words, the calibration information is a calibration table of aplurality of outputs from each of the at least two strain gauges, giventhe various rope tensions and rope angles. And the sets of data pointangle and tension pairs is created from a curve fitted to the createdstrain gauge outputs. It is then determined where the two sets of datapoints intersect. This point is then put back into either of the datesets to determine the rope tension and rope angle.

For a large electric shovel, for example, the resultant tension or forceP and angle of application of this force on the sheave rod will liebetween 34 degrees (max extension and hoist), and 60 degrees (tuck)clockwise from the y-axis of the sheave rod, given the structure of theboom, arm, and dipper.

As an example, a total of 14 calibration curves were obtained for asequence of calibration loads applied between 34 and 60 degrees inincrements of 2 degrees. More particularly, a sequence of calibrationloads was applied at an angle of 36 degrees. At each load increment, thecorresponding voltage output from the x-axis and y-axis bridges wasrecorded. A fifth order curve fit was then determined for the Y and Xaxis data. The sheave rod was then rotated 2 degrees, and anothercalibration loading was applied, and so on.

The fifth order curve, i.e., y=a+b*x+c*x**2+d*x**3+e*x**4, results in anoverall accuracy of 0.1%. The maximum design rope tension for a largeelectric shovel is 2 million pounds. At 1% accuracy, this results in anideal resolution of +/−10 tons, and at 0.1% an ideal resolution of +/−1Ton.

FIG. 6 is an actual calibration curve for an x-axis bridge calibrationloading applied clockwise 42 degrees from the y-axis of the sheave rod.Note the maximum calibration load is 1.05 million pounds.

The difficulty with such calibration data is that as the resultant loadmoved on the sheave rod, the change in output of the sheave rod was notlinear. Along any calibration curve, the accuracy is 0.1%, but off thecurve the accuracy fell to 1%. If we know the angle that the load isapplied and this falls along one of the calibration curves, the sheaverod was very accurate.

FIG. 7 shows a plot of the ratio of an inclined load to a vertical load,as a function of applied angle under ideal conditions. The Load Ratio(E) varies from 1 (vertical load) to 0 (horizontal load). The change inload is nonlinear from 0 to 45 degrees.

The mounted orientation of the sheave rod 38 on the power shovel 8 ischosen to insure that the resultant forces of the hoist rope on thesheave rod 38 were between 30 and 60 degrees for maximum sensitivity andlinearity.

More particularly, the output of each strain gauge 40 in the sheave rod38 in an operating shovel 8 is a millivolt (mV) signal from each of theX and Y bridges. The following procedure is then followed to obtain theresultant force and it angle of application.

From the x-axis bridge output, for each of the calibration curves forthe x-axis, calculate the resultant force given the mV output. Forexample, assume the x-axis bridge output is 0.095 mV. At 34 degrees,this might result in a calculated resultant force of 200,000 lbs. At 36degrees, 280,000 lbs., and so on. We now have 34 calculated resultantforces from each of the x-axis calibration curves.

From the y-axis bridge output, for each of the calibration curves forthe y-axis, calculate the resultant force. For example, say the y-axisbridge output is 0.195 mV. At 34 degrees, this might result in acalculated resultant force of 800,000 lbs. At 36 degrees, 720,000 lbs.,and so on. We now have 34 calculated resultant forces from each of theY-axis calibration curves.

Plotting the above data then produces two curves, the output of y-bridgeand the output of x-bridge as a function of applied load angle. Thepoint where these two curves cross is our unknown resultant force (fromthe y-axis) and the resultant angle (from the x-axis).

Analytically, by fitting a fifth-order curve to each of these two datasets, and setting these two fifth-order equations equal to each otherand solving for the real roots we obtain the angle at the point wherethe two curves cross. Substituting this angle into either of the twofifth-order equations then produces the Resultant force. The accuracy ofthe resultant force is within 0.1%. In other less preferred embodiments(not shown), a second, third or fourth order curve could be used.

Dipper Position

An accurate position of the dipper is needed to calculate the weight ofsoil in the dipper 22. A calibration procedure is performed at thebeginning of operation of the shovel 8, with the dipper 22 in a knownposition. For example, the crowd arm 25 is extended until the hoist rope23 is vertical, and raised until it is horizontal. These orientationsare checked with suitable equipment. From this calibrated position, thecrowd arm length and the hoist rope length are initialized to theirrespective known dimensions as determined from the shovel mechanicaldrawings. Recalibration is needed when any of the wire ropes on theshovel are replaced, or when other shovel conditions change, such aswhen the arm rack and pinion drive system skips a tooth.

More particularly, the resolvers are used to calculate the length of thehoist rope and the length of the crowd arm. From these lengths, thedipper position is found. Once the dipper position is known, as well asthe hoist rope tension and the angle the hoist rope leaves the sheaverod, then the dipper weight can be calculated. Once such known approachto calculating the dipper weight is described in US Chang et al U.S.Pat. No. 6,225,574.

To calculate the hoist and crowd length, the following

equation is used: Length=offset+gain*resolver reading. The gain is knownand constant for both the hoist and crowd resolver and has units ofinches per count. Using the resolver reading from the hoist and crowd atinitial calibration, one can use the above equation to solve for thecalibration offset for hoist and crowd.

Unfortunately, electric shovel operators may forgo recalibration of theoffset when shovel conditions change. And if this happens, then thedipper position is not accurate, and as a result any weight measurementsare also inaccurate. Experience has shown that the hoist offset does notchange very much, but the crowd length offset can. It is thereforeuseful to create a method of automating recalculating the crowd lengthoffset as shovel conditions change. The following method can also beused to recalculate the hoist rope offset, if desired.

Using a microprocessor connected to the crowd length resolver, such asthe one used in the above strain gauge reading mechanism,

a) Keep track of the max crowd reading. From the length of crowd at maxcrowd extension, calculate the offset for the crowd.

b) From one dig cycle; obtain an array of readings from the crowdresolver.

c) At the same time, obtain an array of the calculated rope wrap anglesfrom the analysis of the sheave rod strain gage data.

If a dig cycle has for example 400 data points, this would result in 400calculated rope wrap angles from the resolver data and 400 wrap anglesfrom the rod strain gauge outputs. If the calibration data was perfect,then each data point in the two curves would be equal. Data point 10from the rod data would match the wrap angle calculated from data point10 from the resolver calculation, for example.

d) Use the estimated values calculated in step a) above as a startingvalue. Then vary the crowd calibration offsets and calculate theexpected wrap angle of the rope for the data collected in step a) foreach variation, thus obtaining an array of readings from the crowdresolver to calculate an array of calculated rope wrap angles. Then, toget a new calibrated crowd resolver offset, use the crowd offset thatproduces the minimum amount of error between the two calculated sets ofdata. For example, sum the square of the difference between thecalculated rope wrap angles from the resolver data and the calculatedrope wrap angles from the sheave rod data, to find the crowd offset thatproduces the least amount of error. Still more particularly, use knownnonlinear programming, such as nonlinear minimization to solve for nunknowns with m measurements, to determine the offset value thatminimizes the difference between the calculated rope wrap angles fromthe resolver data and the calculated rope wrap angle from the otherdevice for measuring the rope wrap angles.

Another method for calculating the crowd offset is to use the hoist ropelength together with the rope angle calculated by the above strain gaugemechanism. With the known length of the boom between the dipper arm andthe end of the boom (the hoist sheaves), the length of the hoist ropebetween the end of the boom and the dipper, and the hoist rope wrapangle, the length of the dipper arm can be calculated using basicgeometry. Once the dipper arm length is known, the crowd offset can bedetermined.

Various other features and advantages of the invention will be apparentfrom the following claims.

1. A method, for a shovel including a base, a dipper, a boom pivotallymounted on the base, a hoist rope extending from the base to the dipperand wrapped over a sheave mounted on the end of the boom, and a dipperarm supported the dipper on the end thereof, the dipper arm beingpivotally mounted on the boom, the method automatically recalibratingdipper position information, a arm resolver mounted on the boom andresponsive to record arm extension and retraction, the arm length beingequal to a arm offset plus a arm gain times said arm resolver reading,the method automatically recalibrating dipper position information, themethod including the steps of: storing the maximum values of the crowdresolver, using the known length of the arm at the maximum extensionpositions, calculate the offset value for each resolver maximum value,obtaining an array of readings from the crowd resolver to calculate anarray of calculated rope wrap angles, obtaining an array of rope wrapangles from another device for measuring the rope wrap angles, andselecting a crowd length offset that minimizes the difference betweenthe two sets of calculations.
 2. A method in accordance with claim 1wherein said crowd length offset is selected by minimizing the square ofthe differences between the two sets of values.
 3. A method inaccordance with claim 2 wherein said crowd length offset is selected byusing nonlinear programming to determine the offset value that minimizesthe difference between the calculated rope wrap angles from the resolverdata and the calculated rope wrap angle from the other device formeasuring the rope wrap angles.
 4. A method, for a shovel including abase, a dipper, a boom pivotally mounted on the base, a hoist ropeextending from the base to the dipper and wrapped over a sheave mountedon the end of the boom, and a dipper arm supported the dipper on the endthereof, the dipper arm being pivotally mounted on the boom, the methodautomatically recalibrating dipper position information, a arm resolvermounted on the boom and responsive to record arm extension andretraction, the arm length being equal to a arm offset plus a arm gaintimes said arm resolver reading, a hoist resolver mounted on the shoveland responsive to record hoist rope extension and retraction, the hoistrope length being equal to a hoist rope offset plus a hoist rope gaintimes said hoist rope resolver reading, the method automaticallyrecalibrating dipper position information, the method including thesteps of: obtaining a hoist rope wrap angle from a device for measuringthe rope wrap angle, and then using the rope wrap angle, together withthe known length of the boom between the dipper arm and the end of theboom, and the length of the hoist rope between the end of the boom andthe dipper calculated using the hoist rope resolver, to then calculatethe length of the dipper arm and the arm offset.